Methods of forming an electrode on a nitride semiconductor substrate such as GaN include polishing a substrate, and further removing damage caused by polishing by dry etching and further, carrying out an oxygen plasma treatment, as disclosed in, for example, Japanese Patent No. 2,967,743 and Japanese Patent Application Laid-Open Nos. 2003-347660, 2004-6718 and 2005-268769. JP2005-268769 presents a manufacturing method of depositing an electrode after etching is carried out using gas containing a group IV element such as SiC4 to intentionally dope the group IV element, thereby achieving a low resistance.
Such conventional manufacturing methods, however, do not satisfy both of ohmic characteristics and adhesion. One method is to obtain a low-resistance ohmic characteristics only after performing high heat treatment at 500° C. or higher after deposition of electrode; another one results in high resistance in heat treatment necessary for device assembly even while low-resistance ohmic characteristics are achieved without performing heat treatment; still another one does not achieve sufficient adhesion even while low-resistance ohmic characteristics are achieved.
More specifically, electric characteristics of a front-side device structure formed earlier may be degraded when high heat treatment is necessary. Further, when electric characteristics vary by heat treatment in the device assembly process, assembly may be carried out at low temperatures to prevent this, in which case, however, heat in an operation after the assembly may cause changes with time. Furthermore, devising electrode structure or subjecting the surface to acid treatment in order to improve adhesion may impose limitations on the process or may impair the low resistance having been obtained.